Life upended across the Back Bay

A day after a transformer explosion sent black clouds sweeping through the heart of the city, residents in much of the Back Bay and the Fenway endured a prolonged blackout that left thousands without power, cost local businesses millions of dollars, and forced curtailment of subways and buses, conventions, and college classes.

Hundreds of utility workers from NStar worked through the day to restore electricity, although more than half of the 21,000 businesses and homes that had lost power remained in the dark early Wednesday evening. Utility and government officials pledged that power would be fully restored by 4 a.m. Thursday.